(a) Field of the Invention
The present invention relates to a polymer blend composition and a tunable actuator using the same. More particularly, the present invention relates to a blend composition of a ferroelectric or piezoelectric PVDF-based polymer material and a block copolymer including poly(methylmethacrylate) (hereinafter, referred to as PMMA) miscible with PVDF and a soft polymer having a glass transition temperature below room temperature, and a tunable polymer actuator using the same.
(b) Description of the Related Art
Piezoelectric ceramic materials have been conventionally used as an element for converting electrical energy into mechanical energy, which is usable in robotics, pumps, speakers, disk drives and camera lenses. However, the piezoelectric ceramic materials have the drawbacks of high brittleness and relatively high fabrication costs.
On the contrary, piezoelectric polymer materials have recently gained interest as a material to overcome the above mentioned drawbacks of the ceramic materials, because they have a high power/weight ratio and high efficiency and are very flexible and easy to process. Unlike traditional hydraulic or aerodynamic actuators, piezoelectric polymer materials can be used to design a compact device with no life-limited components such as gears and bearings.
There are piezoelectric polymer materials such as poly(vinylidene fluoride) (hereinafter, referred to as PVDF) and its copolymer poly(vinylidene fluoride)-co-trifluoroethylene, nylon, cyanopolymer, polyurea and polythiourea. In particular, PVDF is flexible, lightweight, tough engineering plastic available in a wide variety of thicknesses and large areas. In addition, since PVDF has the characteristics of having a wide frequency range of 1-10 MHz and converting a minute mechanical stimulation into large electrical signals, the PVDF films are applied to various fields such as switches, computer graphics and electronic games, robot touch sensors, infrared sensors, medical sensors, pickups for musical instruments, and military underwater sound detectors. The piezoelectric property of PVDF is due to beta and gamma crystals, but PVDF is disadvantageous in that it has a significantly smaller piezoelectric constant than most piezoelectric ceramic materials. To improve its piezoelectric property, there is also an inconvenience in that a complex process such as poling and drawing is additionally performed to form beta crystals.
In order to address these problems, PVDF copolymers with trifluoroethylene and tetrafluoroethylene have been prepared to improve its piezoelectric property. Among them, poly(vinylidene fluoride-co-trifluoroethylene) (hereinafter, referred to as PVDF-TrFE) prepared by copolymerization with trifluoroethylene stably crystallizes into the beta crystals without the additional process such as poling and drawing, compared to PVDF. This results in a larger piezoelectric response, and d33 values for PVDF-TrFE is as high as 38 pC/N versus 33 pC/N in pure PVDF. In addition, the materials have a high ferroelectricity (the ability of a material to change its direction of spontaneous polarization in response to application of an electric field), and the highly ferroelectric materials have a wide range of applications such as memory storage elements. However, ferroelectric materials exhibit a hysteresis behavior, making it difficult to control the strain according to applied voltages. The materials are also considerably rigid in themselves, and thus they are needed to be slightly modified for actuator applications.
In order to reduce hysteresis for actuator applications, studies have been made on methods for preparing relaxor ferroelectric materials. At Pennsylvania State University, Qiming Zhang and his research group have fabricated an actuator having more excellent performances by introducing structural defects via high-energy electron irradiation of poly(vinylidene fluoride-trifluoroethylene or by synthesizing a terpolymer with incorporation of a smaller amount of bulky monomers (chlorofluororthylene, hexafluorpropene) into the polymer, thereby increasing a strain (in the thickness direction) of PVDF actuator from 2% up to 7% (Patent literature 1, Non-Patent literature 2).
In order to impart more excellent properties, addition of fillers to the above material has been also studied. However, preparation of composites by the addition of fillers is disadvantageous in that dielectric loss generated by filler aggregation is increased or a dielectric strength and a breakdown voltage are reduced to deteriorate the stability.
To solve these problems, chemical grafting of copper (Cu) phthalocyanine or aniline oligomer is performed during the synthesis step so as to mix the filler ingredients with a polymer matrix at a molecular level, thereby improving actuator performance and minimizing dielectric loss and the reduction in dielectric strength (Non-Patent literature 1).
An actuator including a piezoelectric layer is advantageous in that it is driven by the application of an external electric field to convert electrical energy into mechanical energy very quickly.
Meanwhile, operation of the actuator including a piezoelectric layer is conducted according to an electrostrictive mechanism. When an external electric field is applied to the piezoelectric layer, electronic polarization occurs in the piezoelectrics. Piezoelectric deformations in the molecular chains can be generated by the field-induced polarization. The generated piezoelectric strain (X) is proportional to the applied external electric field (E) and a piezoelectric constant (d) (x=dE). This can be also expressed by a displacement (D) being proportional to the applied stress (X) and the piezoelectric constant (d) (D=dX).
Based on the electrostrictive mechanism, the actuator can be more easily driven by reducing an energy barrier needed for the deformation, for example, introduction of structural defects via high-energy electron irradiation or synthesis of a terpolymer with incorporation of a smaller amount of bulky monomers, accomplished by Qiming Zhang and his research group.
However, the above-mentioned prior art is problematic in that the severe crosslinking side reaction occurs during the high-energy radiation and a complex and laborious chemical process has to be performed for each synthesis and purification of terpolymers or grafted polymers having various compositions in order to control the perfomiance.
Further, Patent literatures 2 and 3 reported that a type or concentration of curing agent, pre-strain or degree of crosslinking is controlled to improve the actuator performance, or a matrix stable in a liquid electrolyte can be developed when used as an ionic actuator. Furthermore, Patent literatures 4 and 5 reported that a PVDF copolymer of P(VDF-CTFE) or P(VDF-TrFE-CTFE) is used to convert the piezoelectric into a relaxor ferroelectric material, thereby reducing the strain hysteresis according to the piezoelectric property. Upto now, however, Not many attention was spotted onto an actuator of which performance is controlled by blending two polymers with different physical properties to have miscibility.